The purposes of the formation of a chromium phosphate conversion coating on the surface of aluminum are to provide corrosion resistance, by enhancing the adhesion of siccative coatings and for esthetic reasons. The conversion coating improves the adhesion of siccative coatings such as paints, inks, and lacquers. A chromium phosphate conversion coating is typically provided by contacting aluminum with an aqueous composition containing hexavalent or trivalent chromium ions, phosphate ions and fluoride ions. Growing concerns exist regarding the pollution effects of the chromate and phosphate discharged into rivers and waterways by such processes. Because of high solubility and the strongly oxidizing character of hexavalent chromium ions, conventional chromate conversion processes require extensive waste treatment procedures to control their discharge. In addition, the disposal of the solid sludge from such waste treatment procedures is a significant problem.
Attempts have been made to produce an acceptable chromate-free conversion coating for aluminum. Chromate-free pretreatment coatings based upon combinations of complex fluoacids and polyacrylic acids which are applied as a single step treatment and dried in place are known in the art. U.S. Pat. No. 4,191,596 which issued to Dollman et al., discloses a composition for coating aluminum which comprises a polyacrylic acid or esters thereof and H.sub.2 ZrF.sub.6, H.sub.2 TiF.sub.6 or H.sub.2 SiF.sub.6. U.S. Pat. No. 4,921,552 which issued to Sander et al., discloses a non-chromate coating for aluminum which is dried in place. The aqueous coating composition consists essentially of more than 8 grams per liter of dihydrohexafluozirconic acid, more than 10 grams per liter of water soluble acrylic acid and homopolymers thereof and more than 0.17 grams per liter of hydrofluoric acid. The disclosure also notes that it was believed copolymers of acrylic acid would also be effective.
Compositions which include an acid and a polymer in a single treatment and which are rinsed to remove excess acid and polymer are known in the art. U.S. Pat. No. 4,136,073 which issued to Muro et al., discloses a composition and process for the treatment of aluminum surfaces using an aqueous acidic bath containing an organic film-forming polymer and a water soluble titanium compound. The disclosed polymers include vinyl polymers and copolymers derived from monomers such as vinyl acetate, vinylidene chloride, vinyl chloride; acrylic polymers and copolymers derived from monomers such as acrylic acid, methacrylic acid, acrylic esters, methacrylic esters and the like; aminoalkyd epoxy, urethane-polyester, styrene and olefin polymers and copolymers; and natural and synthetic rubbers. Treated panels are rinsed with water and dried after immersion in the treatment solutions.
It is also known to remove excess aqueous multiple component acidic solutions from metal surfaces prior to treatment with a polymer. U.K. Patent Application GB 2165165A discloses an aluminum metal surface treatment process comprising sequentially cleaning a metal surface with preferably an alkali cleaner; rinsing the metal surface with water; contacting the metal surface with an aqueous acidic treatment composition; rinsing the metal surface with water; and contacting the metal surface with a post treatment solution containing derivatives of polyalkenylphenol polymers and drying the metal surface. The aqueous acidic treatment composition comprises dissolved metal ions selected from the group consisting of hafnium, zirconium, titanium and mixtures thereof; phosphate ions; fluoride ions; a vegetable tannin compound; and a sequestering agent.